An image display apparatus including a backlight device (surface illuminating device) is used as an image display apparatus for a word processor, a laptop personal computer, etc. In many cases, an edge light type backlight device is used as a surface illuminating device for such an image display apparatus to meet the demand on decreases in weight and thickness. In the edge light type backlight device, a linear light source, such as a fluorescent lamp, is arranged at a side of a transparent plate member (light guide plate), and a display panel is arranged on the light guide plate.
However, as an image display apparatus for television system etc. is recently increased in size, a luminance of an edge light type backlight device may be insufficient. Hence, a direct back light type backlight device (surface illuminating device), in which a linear light source is arranged directly below a display panel, is frequently used (for example, see Japanese Unexamined Patent Application Publication No. 2007-316421).
FIG. 22 is a perspective view showing a brief structure of a direct back light type backlight device 1 of related art. The backlight device 1 includes linear light sources 2 such as fluorescent lamps, a reflector plate 3, and a diffuser plate 4.
The linear light sources 2 may be, for example, cold cathode fluorescent lamps (CCFLs) or the like. The linear light sources 2 have columnar shapes extending in a predetermined direction.
The reflector plate 3 is arranged to reuse light which is reflected by the diffuser plate 4 and the like, and light which is emitted from the linear light sources 2 but does not reach the diffuser plate 4.
The diffuser plate 4 is an optical member, in which resin is randomly contained in a transparent base, the resin having a different refractive index from a refractive index of the transparent base. The diffuser plate 4 has a thickness of at least 1 mm to increase diffusing property and scattering property. The diffuser plate 4 decreases unevenness in front illuminance distribution.
In the backlight device 1, the reflector plate 3 and the diffuser plate 4 are arranged on opposite sides with the linear light sources 2 interposed therebetween.
The image display apparatus having the backlight device is recently increased in size and decreased in thickness. Owing to this, the backlight device is also increased in size and decreased in thickness. The number of linear light sources to be used for the backlight device is increased, and a gap between an optical sheet and the linear light sources is narrowed, the optical sheet being a flat plate arranged directly above the linear light sources.
Unfortunately, when the number of linear light sources to be used is increased, power consumption of the backlight device and the image display apparatus including the backlight device may be increased. Thus, the increase in power consumption may be suppressed by increasing a distance between the adjacent linear light sources, without increasing the number of linear light sources to be used.
If a distance L between the centers of the adjacent linear light sources 2 is increased, referring to FIG. 23, a luminance of irradiation light beams of the backlight device 1 may be increased at positions directly above the linear light sources 2, and decreased at positions between the linear light sources 2. Uniformity of a front luminance distribution is degraded, and the luminance may become uneven.
Even when a distance W between the center of each linear light source 2 and the diffuser plate 4 is decreased to decrease the thickness, regarding the front luminance distribution, the luminance may be increased at the positions directly above the linear light sources 2 and decreased at the positions between the linear light sources 2. Uniformity of a front luminance distribution is degraded, and the luminance may become uneven.
Hence, referring to FIG. 24, there is suggested a method of decreasing the unevenness in luminance by providing a plurality of protrusions 6, having the same aspherical shape, on a light exit surface 5a of a diffuser plate 5, which is formed by dispersing a diffusing material such as a filler in the diffuser plate 5. The method can provide a diffusing effect due to the filler and a diffusing effect due to the aspherical protrusions 6 for light directly above the linear light sources 2. The unevenness in luminance can be decreased.
However, with this method, if the distance L between the centers of the adjacent linear light sources 2 is increased or the distance W between the center of each linear light source 2 and the diffuser plate 5 is decreased, the effect of decreasing the unevenness in luminance may be degraded (see FIG. 24). It is difficult to increase the size and decrease the thickness of the image display apparatus while suppressing the increase in power consumption.
Alternatively, there is a method of increasing an amount of return light at positions directly above the linear light sources 2 by using a diffuser plate 8 having protrusions 7 each having a substantially prism shape, instead of the diffuser plate 5 having the aspherical protrusions 6.
However, with the diffuser plate 8, if the distance L between the centers of the adjacent linear light sources 2 is increased and the distance W between the center of each linear light source 2 and the diffuser plate 8 is decreased, referring to FIG. 25, unevenness in brightness every ½ period may appear such that a narrow area directly above the linear light source 2 is dark, an area next to the narrow area is bright, and a substantially middle area between the linear light sources 2 is dark.
In light of the situation, it is desirable to provide a surface illuminating device and an image display apparatus capable of decreasing unevenness in luminance without increasing power consumption.